1. Field of the Invention
The present invention relates to the lithographic exposure and patterning of workpieces, and in particular relates to a mechanical method and apparatus for selectively masking a workpiece to prevent exposure of select regions on the workpiece.
2. Description of the Prior Art
Lithography techniques are used in the manufacturing of microdevices, such as integrated circuits (ICs), flat panel displays, microelectromechanical systems (MEMS), the formation of bump IC interconnects for xe2x80x9cflip chipxe2x80x9d interconnection technology, and the like. The lithographic process involves the use of photosensitive workpieces (xe2x80x9cwafersxe2x80x9d) and the selective exposure of such workpieces with radiation (e.g., UV light). Workpiece photosensitivity is typically achieved by coating or otherwise applying a layer of photosensitive material called photoresist to the workpiece surface. Photoresist can either be xe2x80x9cpositive-tonexe2x80x9d in which the exposed resist is removed upon developing, or xe2x80x9cnegative-tonexe2x80x9d in which the unexposed resist is removed upon developing.
Generally, the lithography process includes the steps of coating the workpiece with photoresist, exposing the photoresist with the image of a mask to form a latent pattern in the photoresist, developing the photoresist to form a three dimensional image, etching the photoresist to form a corresponding three dimensional pattern in the workpiece, and then removing the excess photoresist. These steps are repeated a requisite number of times to form the particular device structure in the workpiece.
In certain lithography applications involving negative photoresist, it is preferred that select regions of the workpiece outside of the individual exposure fields remain completely unexposed so that the resist in these regions is removed upon developing. One example application is bump interconnect lithography, which involves the formation of conductive (e.g., a gold or solder alloy or other suitable metal) bumps on a workpiece (wafer) that are used to contact circuit lines on a circuit board. Bump lithography includes an electrochemical plating step to form the conductive bumps. This step requires contacting most of the edge of the wafer with an electrode. Further, the thickness of the resist layer used in bump lithography is relatively thick (e.g., 50 microns) as compared to so-called xe2x80x9cfront-endxe2x80x9d lithography. For bump lithography, substantially all of the edge of the wafer must be free of photoresist to ensure a uniform electrical contact.
When one-to-one contact or near-contact photolithography is performed, the entire wafer is exposed at once and the exclusion of the edge is achieved by incorporating the desired exclusion region into the reticle. Contact or near contact lithography for 200 or 300 mm wafers is limited by defects and large, expensive masks and mask contamination. Step-and-repeat lithography eliminates these disadvantages, though in many instances the exposure overlaps the edge of the wafer in situations where it is preferable not to expose at or near the edge of the wafer. In step and repeat photolithography, however, it is not possible to define an exclusion area into the reticle since the stepping pattern depends on the pattern size and the step sizes. There is therefore a need for a method and apparatus for defining an exposure exclusion region on a workpiece for certain photolithographic processes. The present invention provides such a method and apparatus.
The present invention relates to the lithographic exposure and patterning of workpieces using either reduction or 1xc3x97 step and repeat lithography, and in particular relates to a mechanical method and apparatus for selectively masking a workpiece to prevent exposure of select regions on the workpiece.
A first aspect of the present invention is method of forming one or more exposure exclusion regions on a photosensitive workpiece to be exposed with an imaging system capable of forming microscopic features on the workpiece. The method includes selecting one or more macroscopic regions of the photosensitive workpiece surface to remain unexposed, and then masking the one or more select macroscopic regions of the workpiece. This is accomplished by arranging a mask proximate (i.e., on or near) the workpiece surface, the mask being opaque to a wavelength of radiation that activates the photosensitive workpiece. After exposing the workpiece, the mask is removed and the workpiece processed. When negative photoresist is used, the portion of the photoresist associated with the exposure exclusion region is removed upon processing so that the surface of the workpiece is accessible.
A second aspect of the present invention is an apparatus for forming one or more select exposure exclusion regions on a photosensitive workpiece. The apparatus includes a mask opaque to the wavelengths of radiation that activate the photosensitive workpiece. The mask is arranged proximate the photosensitive workpiece so as to cover select macroscopic regions of the workpiece. In a preferred embodiment, the mask is a ring sized to cover an (annular) region of the workpiece adjacent an edge of the workpiece. The apparatus may further include a mask handling system adapted to deliver the mask to and from the workpiece.